Optical inspection systems for distinguishing between the epoxy substrate and copper tracks of a PCB are well known. Thus, for example, U.S. Pat. No. 4,152,723 (McMahon et al.) describes a method and apparatus for generating a digital image representation of a printed circuit board having a substrate of insulated material and a pattern of metallic conductors disposed on a surface thereof. A beam of light energy scans the surface in a predetermined pattern so as to induce a detectable fluorescence in the surface of the insulating material. The fluorescence is detected and a binary signal is generated to indicate the presence or absence of fluorescence as the beam scans the surface.
Typically, a Helium-Cadmium laser is used which emits a blue beam resulting in a relatively strong yellow fluorescence when the laser beam is incident on the PCB substrate. A filter allows the yellow fluorescence to pass therethrough but blocks the blue laser beam reflected from the copper tracks. The collected fluorescence is detected and analyzed in order to provide an accurate representation of the PCB's surface despite the fact that etched conductive foils often have irregular surfaces.
In fact, since the fluorescent signal is emitted only when the substrate is struck by the laser beam, such a system provides a good indication of the presence or absence of copper track on the insulating substrate but gives no indication relating to the surface quality of the copper track itself. It is therefore known to collect the laser beam reflected from the copper track in addition to the fluorescent emission produced by the insulating substrate, thereby not only to distinguish between the substrate and copper track but, moreover, to analyze surface irregularities within the copper track itself.
Thus, for example, U.S. Pat. No. 4,556,903 (Blitchington et al.) discloses such a system wherein respective fluorescence and reflected beams are produced when a laser beam strikes the substrate and the copper track of a printed circuit board. In the system described by Blitchington et al., both signals are collected, after suitable filtering, so that analysis of the copper track itself may also be undertaken. In particular, such a system permits the uniformity of the copper conductors to be established.
However, in practice, there are many other possible deformities associated with the copper tracks which a practical inspection system must address. PCBs commonly include several layers of conductive track within an insulating substrate, electrical conduction between one or more of the so-called "multilayers" being effected via plated through-holes or "vias" drilled through the PCB. In such a PCB, the major types of defects include:
(a) shorts and surface shorts between conductors; PA1 (b) cuts in conductors; PA1 (c) pin-holes in conductors; PA1 (d) conductor width violation: (comprising local violation, i.e. nick-down or "mouse bites" as well as global width violations); PA1 (e) insulator width violation; and PA1 (f) defects in the conductive tracks which do not penetrate through the complete height of the conductive track down to the laminate, but merely reduce the depth of the conductor (known as "dish-down"). PA1 (a) hole incompletely drilled; PA1 (b) breakage in hole sleeve; and PA1 (c) hole incorrectly centered relative to a surrounding solder pad.
Additionally, there exist three major types of defect related to the plated through-holes, or vias, these being:
It is known that systems which rely on the reflection method alone produce many false alarms since it is difficult to detect all of the defects in a PCB owing to the oxidation of the conductive tracks and the poor reflection from very narrow shorts. This problem led to the development of the systems described above which distinguish between the conductive track and the laminate by means of the fluorescence produced by the laminate consequent to its illumination by a specified laser light source.
On the other hand, there are features such as dish-down, wherein the conductive track is not completely eroded but is of reduced thickness, which can only be detected based on the reflection of light therefrom.
Additionally, as was explained above, there currently exists no satisfactory method for inspecting imperfections in the plated through-holes, particularly in combination with the fluorescence and reflection based systems of the type described.